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CHAPTER

Thermal Analysis and Kineticso f Degradation

--. --?hurts presenteS in tfjk cljapter Gave h e n communicateS to:

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9.1 IntroductionT ermogravimetry offers precise control of heating conditions, such asvariable temperature range and accurate heating rate. This analysisneeds only a sniall quantity of sample. It is also possible to quanhfy theamount of moisture and volatiles present in the composites, which have adeteriorating, effect on the properties.l.2 The thermogravimetric dataprovides number of stages of thermal breakdown, weight loss of the materialin each stage, threshold temperature, etc.3 Both TG and DIG (differentialthermogravinietric) curves provide information about the nature andconditions of degradation of the material.

TGA can help in understanding the degradation mechanism and mustassist any effort to enhance the thermal stability of a polymeric material4. Thethreshold decomposition temperature of composite indicates the fabricationtemperature. The thermal stability of individual polymers can be evaluatedto a great extent by blending it with other polyniers or reinforcing with fibres.Composites usually have better thermal properties than correspondingcomponents in the system5.6. This synergism is usuaUy attributed to theinterface adhesion of the components.

'l'he thermal degradation of natural fibres has rcccivcd considerableattention in the past'8. The effects of crystallinity, orientation and

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crosslinking on the pyrolytic behaviour of cellulose fibre have already beendescribed in considerable detail9-12. It is observed that the thermal breakdownof cellulose proceeds essentially through two types of reactions. At lowtemperature@etween 120C and 250C), a gradual degradation, whichincludes depolymerization, hydrolysis, oxidation, dehydration anddecarboxylation. At higher temperature, rapid volatilization occursaccompanied by the formation of levoglucosan and a charred product.Decomposition leads to loss in fibre strength and a marked reduction indegree of polyn~erisatiot~(l)I').nitial n~olecularweight decrease is severe andoccurs via rupture of chains at the crystaLline/amorphous interfaceI3. Thepyrolysis of lignin has been studied by Abet4. Thermal stability and moistureretention of coir fibre has been increased by the effect of mercerisationl5.Mohanty c t nl1f1. have studied the thermal analysis of PAN(po1y acrylonitrile)grafted coir fibres. Thus TGA studies estimate the thermal stability of thefibresl7.

Various researchers have previously studied the thermal behaviour ofrubber blends and composites in detail'"l9. 'lhe storage stability of somesynthetic rubbers and ENR(epoxidised natural rubber) has been analysedthrough TGA by Chaki et alls. TGA has been used to determine the fillercontent of wood-based compositesl9.

DSC (differential scanning calorimetry) can explore the heterogeneousnature of rubber composites. This mil l give the Tg of the polymeric materialand the fibre phases. Miscible system will show single, sharp transitionpeak(T,) intcrrnediate between those of the components. Separate peaks willbe obtained for heterogeneous phase separated systems.

In this laboratory, Thomas and co-workers have already reported onthe- t h r r m n l an,rlp.;is of rr~bl>er lencts and compositcs"'?2 irr detail. Veryrecently Prasantha Kunrar and 'Ihomas have prepared cost-effectivecomposite n~aterialsrom styrene-butadiene rubber and sisal fibre ?'.IJ.

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In this cliaptrr, the thermal behaviour of the sisal fibre rcinforced SBRcomposites has hc.c.11 an'ilyscd by 'I'GA and USC as a tunction of fibre loading,chemical modification of fibres and incorporation of bonding agent. Attemptshave also bccn made to corrclatc the properties with thc morphology of thesystem. Finally, the kinctic paramctcrs of thc thermal degradation wcrc aLsoevaluated using ninc mechanistic equations.

9.2 Results and discussion

9.2.1 Thermogravimetric Analysis(a) 'I'herma! analysis of fibres

The major constituents of sisal fibres are cellulose, hemicellulose,pectins, lignin, etc. Of these, cellulose is the main fraction reprtwnting 72% ofthe fibre followed by hemicellulose and pectins (10%)'6. The differences in theproportions of ccllulose, hemicellulose and pectins in untreated and treatedsisal fibres contribute to the thermal characteristics of the sample. Scheme 9.1shows the structure of cellulose backbone present in the fibre.

Sc!:cme 9.1. Strucfure of cellulose bnckhorte i r r thejibre.

Thermal decomposition of each sample takes place in between aprogrsamec! temperature range of 10 to 550'C. Most natural fibres lose theirstrength at about 1 6 K . Thermal analysis of cellulose fibres has been carriedout and the cfffcts of cl-ystallinit)., orientation and crosslinking on the

' - 7pyrolytic behaviour of cellulose have been reported2 -* -'.

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Figure 9.1 shows the 7'G curves of untreated and chemically modifiedsisal fibre. Curve A represents untreated, B, mercerised, C, acetylated and D,benzoylated sisal fibre respectively. 7'he major source of stability in celluloseis due to hydrogen bonding, which allows thermal energy to be distributedover many bonds3L1. Between 75 to 1750C dehydration as well as degradationof lignin occurs and most of the cellulose is deconiposed at a temperature of350C. 'l'he oxidative degradation of cellulose takes place in the aulorphousregion7. The decomposition of treated fibres occurs at a temperature range of460-472"C, whereas the untreated fibre decou~posed t 45:;.7Y;;C. 'l'able 9.1shows the mass losses (76) of untreated, mercerised, acetylated andbenzoylated sisal fibre at various temperatures. On analysing the table, it isfound that the decomposition temperature of the treated fibre is higher thanthat of untreated fibre.

100 2 00 3m un s inTEMPERATURE ( 'CI

Figure 9.1. 7.G crrnVesf untrenled, rrrerceri.~errince~~l(iredrrri brrizo.~lnld isnlfibre

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Table 9.1. I l i ~ i g h fasses of ttrtlreared, ntercerised, acefylafed artd berizo~latedsisal fibre a f various temperatures,

Figure 9.2 shows the DTG curves of the untreated, rnercerised,acetylated and benzovlated fibre. The letter A corresponds to untreated, B,mercerised, C, acetylated and D, benzoylated sisal fibre respectively. Thecomplete pyrolysis of the fibre sanlple takes place in t h e e stages showing avery small peak, a broad peak and a sharp peak. The thennogram of sisalfibre obtained by Chand e t al?' have also revealed that its thermaldegradation follows three distinct stages. Ln the case of untreated fibre, thelirsl pmk L > C ~ M . C ~ - I I h0 811d IOO'C C ( I I . I . ( ' S ~ O I ~ ~ So thr 11c.at ol vaporisalio~~twater in the sample. The second peak at about 325" is due to the thermaldepolymerisation of hrnlicellulose and the cleavage of glucosidic linkages ofcellulose3'. The third peak in these thermograms was sharp and at about450'C may be due to t h r further brcakogr of the dccc i r~~~~os i ic t~roc1uctsfstage 11, leading to the formation of tar through levoglucosan".

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Figure 9.2. D I G cunqes f l urtireared, ntercerised, nce!flnted nrtd benzojqlnIedsisalJibre

The main dcconiposition temperatures of chen~icallymodified fibre areshifted to the lugher side with respect to untreated fibre, i.e., the DTG peaktemperature at 45PC in the case of untreated fibre is shifted to 465-472'Crange in the case of t~eated ibre. I'he mass loss(%) of treated fibres at thisstep(combination of heniicellulose & a-cellulose) increases, whereas the charleft(%) at 475'C showed a decrease. I'he percentage of char is higher in thecase of untreated fibres ('I'able 9.2). 'The rate of thermal degradation betweenouter and inner layers of the cellulose fibrils it1 fibre particles present insamples may be significantly different. Consequently, complete pyrolysistakes placc in a relatively wider temperature band showil~g a broaddecompositiori peaks. On degradation, ITI'G curves displace towards hightemperature side depending on the alnount of residual non-cellulose

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polysacharicies present with the cellulose fibre"'. l 'he physical differencesbetween the fibre c a n be clearly identified from their cross-sections and alsofrom their surface cl-lar-acteristics.For cxaniple , trcatcd fibres sl-tow a roughsurface topography compared to untreated fibres. S E M Figure 9.321 shows themorphology of the untreated fibre while the Figure 9.3b shows the fibre aftertreatment. The size and shape of the test sample can also affect the pyrolysispatterns due to the diFferences in surface area and heat conductivity.3";rhble 9.2. Results of derivntive rhernzogravinlctric nttaZysis of sisnL n ~ t dreated

sisalf ibre.1 1 Jegradation) 1 I Residual 1eight loss(%)Sisal f ibre temperature ttC) mass at 47S0C(at cliff.-Peaks (Peak temp)- - ---- -- (%)1 Untreated fibre 1 335.49 1 51.82 1 9.228 1

Figure 9.3. SEII~l torugrplrs shotuirrg th e scrrficr ntorphology of (4 raw s i s ~ lfibre arid (6 ) b e n z ~ ~ l n t e d f i b r c .

hlercerised-Acetylated

---Renzoylated

Chapter 9:Thermal and Kinetic Degradation 266

342.46465.3245.8

343.72468.5947.41345.45471.85

48.159.2568.2748.656.6366.249.522.72

4 .(A

3.076.-

2.307

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The chemical treatment on sisal fibres reduces the pyrolysistemperature, decreases the weight loss during pyrolysis facilitating theformation of a lower percentage of flammable volatiles. Table 9.2 also showsthe results of DTG runs of untreated and treated sisal fibre. The differences inash left in the crucible at the end of the DTG runs have shown that theinorganic content of each of the three treated samples of fibre was notsignificant. Maxittiurn ash was detected in the case of untreated fibresxi. Ingeneral, alkali treatment and mercerisation of natural fibres reduce thecementing material followed by the removal of volahle products with ruptureof bonds. It is well estabkh ed that thermal behaviour of cellulose fibres islargely influenced by alkali treatment, which also affects their mechanicalproper ties's.

Treated sisal fibres would have more surface area compared withuntreated fibres for the same weight assuming that density of these fibres issimilar. ?'he pyrolysis would advance rapidly in treated fibres and the rate ofdecomposition may be xelated to mean radius of the fibre. This conclusion isfurther supported by the parameters determined by 1)'I'G. DifferentialThermogravimetric Analysis not only provides a thermal spectrum of thesample but may also be able to relate to the treatment of the fibre. UsingTGA, Kokta et a l . 5 have studied the effect of grafting of various polyacrylateson to cellulose and its influence on temperature and heat of degradation.

DTG therrnograms of untreated and treated fibre show initial moistureloss peak between 60 a n d 100C nd a main deconlposition peak at 325'C foruntreated fibre, 342.6OC for mercerised, 343.720C for acetylated and 345.45Cfor benzoylatecl fibre. In the case of treated fibre, the decomposition step due

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to the hemicellulose, which is observed in the case of untreated fibre, is almostmerged with a-cellulose degradation.

The weight losses of these treated and untreated sisal fibres at varioustemperatures are given in Table 9.1 In the case of untreated cellulose fibres,lignin degradationH ets in at around 200cC, and other polysacharides mainlycellulose are oxiciised and degraded at higher temperatures?2. From the data,it is clear that benzoylated sisal fibre exhibits a higher thermal stabilitycon~pared o other treated fibres. This may be attributed to substitution ofbulky phenyl group in the fibre that restricts the segmental mobility, therebyincreasing the stiffness of the cellulose backbone. It is also partially due to thefact that some of the components of the fibre, such as LignocelIulose, thatdegrade at a lower ten~peraturemay be extracted during alkali treatment.The improvement in the thermal stability of the fibre will lead to the betterservice perfornlance of the composites at elevated temperatures.

(b) Thermal analysis of composites'Ihe formulation of the mixes are given in Table 9.3. Figure 9.4 shows

the thermal degradation of gum SBR sample. On analysing the TG curve ofthc sample, it is found that the dc,composition of Sllli occurs at a temperatureof 445.17C. The Sf3li starts degrading at 414.09.Cand only 2.889% emains at60OC. I'he decomposition temperature of SUR gun1 is higher than that of thefibre. DTG curve of the sample showed two peaks. l 'he degradation of SBRstarted from the minor peak and followed by a broad decomposition peak.

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Table 9.3. Fornrulatior~ f m ire sfrom A to S@ lrr)-Mixeshgredients

A B C D E F G H L Q R S--Synaprene lo o 100 100 1 0 0 ~ 0 0 00 1 0 0 ~ ~ 0 000 loo 100 100Sulphur 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2 2 2.2 2.2 2.2Stearic Acid 2 2 2 2 2 2 2 2 2 2 2 2Zinc Oxide 5 5 5 5 5 5 5 5 5 5 5 5

Hexa* - - - - - - . - 2.5 - -

Sisal fibre(Fibrelength,mrn) - 2 6 1 0 6 6 6 6 6 6 6 6Sisal Fibre(Untreated) - 35 35 35 5 10 15 20 35 - - -

(Acetylated) - - - - - - . - - 35 -+ Ilexamethylene Tetramine*f N-cyclo hexyl benzothiazole sulphenamide'" ,2,PTrimethy 1,2-di hydroxy quinoline polymerised.

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Figure 9.4. TGA attd DTG curves of vuicarrized SBR (Mix A)The degradation of SBR occurs in two steps. It is stable up to 250C

and after that degradation occurs rapidly. ?he degradation starts at about414.0YoCand at 5 5 K lmost 92% degradation is completed. Above 7 5 0 C noresidue remains. In the D'I'G of SBR, a major peak is observed at 501.370C,which indicates the degradation of saturated and unsaturated carbon atomsUI S U R . l'hiscorresponds to the chain scission to give volatile monomer alongwith some amount of impurities. The mechanism of random scission ofpolymer chains that occurs at high temperature is shown in Scheme 9.2.

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- = I + c n ) + ~ ~ ~ - " + = - = c ~ - - F * a -alSchente 9.2. Degradation sites of SBR.

The TG and DI'G curves for the fibre filled(17.7%) mix C is shown inFigure 9.5. The degradation temperature of fibre reinforced composites isgreater than the vulcanized SBR matrix. The fibre filled SBR starts degradingat 300C and 6-7% remains at 600c.C. About 80% of the material gets degradedin the temperature range of 400-60O.C. lliis is evident from thethermogravimetric scan that the thermal stability of fibre reinforcedcomposite is increased. DTG curves also supports this fact.

Ts*oll"*T""E , C,

Figure 9.5. TGA and DTG curves of n ~ i v (35PIir)

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In sisal/SUR composites, generally two peaks are obtained. In the caseof untreated fibre reidorced SBli composites(Mix C), the degradation ischaracterised by three peaks. The minor peak at 386.36'C corresponds to thedegradation peak of SBR and the major peak at 494.09"C corresponds to thedegradation of sisal/SBR composite. The third peak at 610.82- indicates theconversion of volatile mass and impurities to the tany products. It isinteresting to note that, in the composite, the major peak is shifted to highertemperature region compared to sisal and SUR peaks, i.e., thermal stability ismarginally increased in the composite compared to pure SBR due toimproved fibre-matrix interaction. Ih is can be further understood from theweight losses at a definite temperature (Table 9.4). In most temperatures, theweight losses are lower in the case of composite samples.Table 9.4. 7fiernral degradation of sisaUSBR conrposites at diJferenttemperatures.

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9.2-2 Thermal decom position as a function of fibre loadingFigures 9.5-9.9 show the 'l'G/Ul'G curves of mix C, E, F, G and H

respectively. On comparing these thermograms, it is understood that theincrease in the volume percent of short sisal fibres has a retarding effect onthe extent of degradation of sisal/SBR composites. On increasing the volumepercent of fibres in the composite, the degradation temperature also increasesas indicated by the 1;igures9.6-9.9. L he resistance to degradation is causeddu e to the presence of increased fibre content and increased interactionbetween the fibre and the rubber matrix. Our earlier studies indicated thatthe interaction between fibre and rubber increases with the increase of fibreconcentration23.

-

.................. ..........................+\........ ,. \ , \1

i ,- , m* ,, 4- , :E: , ,5 4Y ,50 : ;, ,,

, .,I :, ,

2, : 'I-

? L I L40 0 ,no >m I," 3%" , c r : \ i i - - ; , 6 7 4 r - - . l ,

Figure 9.6. 1'GA atui DI'G cunvesofrrrir E(5Pltr).

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.~1*,.11111111"~, I C,

Figure 9.7. TCA and DTG cunresof ntiv F(I0Pllr).

Figure 9.8. TCA arrd D7% curves of miu G(I5Plrr).

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Figure 9.9. TG A arrd DTG curves of nriv H(20P11r).The parameters evaluated from the thermograms and their derivatives

of sisal/Sl3Ii composites are tabulated and shown in 'l'able 9.5. 'I'he initialdecomposition temperature ('1'4,emperature for 50% decomposition ('150)andthe final decomposition temperature (TI)were plotted against volume percentof fibres (Figure 9.10a). It is seen that fibres increase the initial degradationtemperature. As the fibre content increases from the mix E to H, the stiffnessand strength of the composite also increase as observed from the mechanicalproperty measurements. The increase in the decon~position emperature isdue to the increased fibre-matrix interaction in the composite, which enhancesthe overall thermal stability of the composites. Figure 9.10b shows the effectof different treatments on the degradation temperature of samples. ?'heincrease of degradation temperature is attributed to the strong interfacecreated through chemical modification of fibres. Among these samples, it isevident that benzoylated sample has the maximum thermal stability thanothers.

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Tab le 9.5. Pnranceters evaluated from tlre tlternwgram s of tlte sisaVSBRconlposiles.FinalVolume

Temp.place('C).

~~

486.664--5.779 358.53 376.61 503.298.425 406.79 498.184 558.21

488.838 588.13 487.047386.359 494.0875 635.53

.--

VOLUME OF FIBRES(% )

k'igure 9. I Oa. /'lo ts of Tb I;" attd Tf ersus voluttte percetrt o ffibres.

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Untreated Bondg Agt Mercerised Acetylated BenzoylatedTYPES OF TREATMENT

Figure 9.10b. l'lols of T;, Tsoarrd TI versus drferertt cl~emicalreatment.The weight loss uf con~posites t different temperatures is tabulated in

'Table 9.4. It is obvious that thermal stability increases in the fibre fdledsystems on increasing th e fibre content due to the belter fibre-matrixinteraction. I t is also understood that the modiEication of fibres by chemicaltreatment has also helped in the iuiyrovenlent of inlel.facia1 adhesion wluc11paved the way fo r the better thermal stabdity .9.2.3 Effect ofchemical treatment 8 bonding agent

The effect of chemical treatment on fibres and incorporation of bondingagent on the system affect the thermal degradation of sisal/SHli composites.'l'his is well explained in the '17G/LY1'G curves of mixes C, Q, I< and S, whichare shown in Figures '3.5, 9.11-9.13respectively. 'l'he cxtcnt of degradation of

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sisal/SBR composites treated with mercerised, acetylated and benzoylatedfibres are less than that of untreated fibre composites. When comparing theweight losses at 450C the values are 45.01% for the untreated composite,39.29%,30.72%, 26.71% for mercerised, acetylated and benzoylated fibrecomposites, respectively (Table 9.4). ?he slightly higher thermal stability oftreated fibre composite can be explained by the additional intermolecularbonding between fibre an d matrix induced due to the treatment. Thebenzoylation of the fibre makes the composites thermally more stable than theuntreated one. This is associated with the better interaction between fibre andmatrix, due to the formation of covalent bonds between GHs-CO- group ofbenzoyl chloride and -OH gtoup of cellulose. The schematic model of theinterface of the benzoylated fibre con~positess represellled in Schentc 4.3

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TUYPII."TII"II I CI

Figure 9.12. TC A arrd DT C cunvesof nliv R.

,,,H,~,z,,hr,,",, ,.,

Figure 9.13. 7%A and 177% cunveso f n ~ i v .

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Figure '9.14 shows the thermogram of bonding agent incorporatedcomposite (Mix L). On analysing the two-stage decomposition as shown byIXG, it is seen that the rubber phase started decomposition at 338C while thecomposite decomposes at a higher temperature shown by the degradationpeak at 525.86"C. liere, the incorporation of bonding agent shows littleenhancement on thermal stability. This is due to the easy degradation of theinterface of bonding agent added composite. ?his is also evident from thevalues of weight loss reported in Iable 9.4. The formation of the ln sihc resinformed during vulcanization is also depicted in Scheme 4.6. Among thesemixes, benzoylated fibre reinforced SnR composite(Mix S) showed maximumthermal stability.

l'7glcre 9.14. TG,? arrd DTG' cunvs c ~ r r r i u*.

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9.2.4 Kinetic parameters from thermal degradationNowadays, tliermoanalytical techniques are extensively used for the

evaluation of kinetic ynrameters, viz., rate constant(k), energy of activation(E),order of reaction(n), pre-exponential factor(A), etc., of heterogeneousreactions. Mass measurement has higher accuracy and precision as comparedto the measurement of bT(DTA) or dH/dt(DSC), and therefore, 'I'G ispreferred in many studies. The general approach in kinetic analysis is toobtain an equation to express the rate of reaction in terms of the degree oftransformation, a , and a temperature dependent function k(T). If the reactionis proven as iso-kinetic over the range of temperature, the second termbecomes the rate constant governed by the Arrhenius law. Mechanism-non-invoking method is a simple extension of homogeneous kinetics, assumingg(u) = (I-up.

A plot of Ln[g(a)/T*] against 1 /T gives a straight lime with slope asE/R and intercept as in AR/$E as shown in Figure 9.15.

Figure 9.15. Plots of lrr 1 y(a)/Id versrrs In:

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Different authors have derived several equations based on the integralmethod for the deterinination of the decomposition kinetics. Using theintegral method explained by Ninan2 on the above rate equation, the kineticparameters for thernial decomposition of sisal/SBI< composites were analysedby the least square technique. ?'hisfits well with the first order kinetics.

'l he mechanisms of kinetic equations are based on the assumption thatthe form of g(a) depends on the reaction mechanism. 'The heterogeneousprocesses are classified into three broad categories governed by (1)transportof matter(diffusion), (2) nucleation and growth of nuclei and (3) phaseboundary reactions. l h e most prominent rate controlling process operating ina particular case is chosen and used for deriving the rate equation. Differentauthors basrd on the integral method to determine t h c , ki~~et icaralnelcrs ofthermal decon~positionhave derived several mechanistic equations. Satwahas chosen 9 equations based on 9 possible mechanisms [9 different forms ofg(a11~~.

'l'he equations and the rate controlling process in each case are shownin Table 9.6 and were used to analyse all TG data. 'l he form of g(a) whichbest represents the experimental data gives the proper mechanism. ?heresults of the analysis have revealed that the highest correlation coefficientand hence best linear fit of the experimental data was obtained from Manlpellequation irrespective of the composition and method of preparation of thecomposites.

The analysis of thermal decomposition data showed that the best-fitcurve was obtained from Mampell equation irrespective of fibre loading andtreatment. ?h is leads to the conclusion that the degradation of sisal/SBRcomposites is based on random nucleation meclianisrn, which is the ratecontrolling process

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Table 9.6. Conrtrmr~ly sed ~(a)o m s for solids state rmclions.I I 1 I- .-.--Equation Rate controlling Process

--.- ~

particle- Mampell equation.Random nucleation- one nucleus on each

Phase boundary reaction, cylindricall-(l-a)1/2 symmetry. -. ....~- 1I 3 l T'hase boundary reaction, sphericall-(l-a)1/3 symmetry. In + (1-a) ln(1-a) Two-dimensional diffusion.

-~

Three-dinlensional diffusion, spherical1 - ) - ( - a ) symmetry- Ginsthing-Brounshtein

equation.~- ~ ~ .. . . ~

Randonl nucleatio~l-Avrami quation 11 7 1 [ - h ( ~ - ~ : , ] ~ / ~andom nucleation-Avrami equation I1- .. - ~ ~ ~ ~ . ~ ~ ~ . ~ ~-

Three-dimensional diffusion, sphericalsymmetry- Jander ecluation.. - .~ .One-djmei~sior~aliffusion.

'I'he kinetic parameters such as E, energy of activation(kJ/mol), A,Arrhenius parameter(S'), and AS ,entropy of activationu/deg/mol) for thetwo-stage thermal decomposition of sisal/SHlZ co~npositesan~pleswithdifferent volumc c ~ f ibres are sun~tnarised n ' lab lrs 9.7 anci 9.8. 'l'heactivation energy, defined as the amount of energy required to decomposeone-gram mole of the substance, also reveals information about thermalstability. The larger the activation energy, the more stable is the material. 'I'heactivation energy of material is dependent on the order of reaction andtemperature range of pyrolysis. But no absolute value of activation energycan be expected in the composites due to its heterogeneous nature.

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Table 9.7. EJSrct of ~~ulumef Jibres (%) on k ittetic paranleters for tlte ilterntaldecot~tpositiort for the b es tfi l equation).

Table 9.7 shows the effect of volume of fibres on the kinetic parameterswhile Table 9.8 shows the effect of treahnent on the kinetic parameters ofthermal degradation. In all cases, the activation energy of the first stage islower than that of second stage. I h e activation energies increase gra~iuallynboth stages as the fibre content increases. In the first stage the activationenergy itlcreases from 204-221 kJ/mol as the fibre content increases from5-35phr. Similarly, the second stage, the activation energy increases from221-284 kJ/mol as the fibre content increases. This shows that the increase infibre content makes the system more resistant and less temperature sensitiveduring thermal degradation.

'Ihe kinet~c tudy of thermal decomposition showed the presence oftwo activation energy values for each composite indicating phase separation.Each phase decouiposes separately at two different teniperatures. 'I he lowactivation energy of first stage of decomposition compared to that of secondstage shows that S15K degrades at lower temperature and fibre degrades at ahigher temperature. The low value of pre-exponential factor indicatesrelatively few degradation sites for the compound.

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Table 9.8. Effect of chemical treatment and bonding agent in sisaL/SBR composites on kirtetic parameters for tlte tltermaldecompositiort (for tlte best fit equation ).7 Entropy of activation, AS1 Activation energy, B (kJ/moi) 1 Arrhenius parameter, A ( ~ ~ 1 )Tvpes of treatments Ia, (J/deg/mol)C-Untreated(N)Q-hfercerised(C)R-Ac.etvlate~l(G)

Stage I221.92223.31242.52

Stage I1284.41287.45307.67

1 307.63-Renzoviated(1) , 336.78299.36-Bondg.lgent(L)

Stage I11283.57210.82

253.46332.51 1 4.73~108

I/ 1.19~10'

Stage I2.6x1W1.12~10'

180.02 1 1.89~10'9.23~1071.03xlW

Stage I14.51xlW6.06x10h2.39~107

5.39~106 -100- 1 -175

3.97x102 -194-86-117

Stage I11 I Stage I

-124 11Stage U

-125-111

3.89x10jStage 111

-145-188

-122

-225

-2022.33x107 / -209

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All values of entropy of activation have negative values. The negativevalues suggest that the segments undergo some chain alignment in elevatedtemperatures forming the activated complex. It is also observed that themagnitude of the entropy change is lower in the second stage of degradation.

'l'he effrct of treatment on kinetic paramctc.rs is made clear fromTable 9.8. It is seen that treated co~nposites how three-stage decompositionshowing th e e different peaks in the DI'G run. l'he first one is a small peakshowing the initial degradation and the second is a broad one showing maindecomposition peak while the last one shows the deterioration of compositeinterface. From the first stage to the second stage the activation energyincreases from 223.31 to 336.78 kJ/mr)l as the fibre treatment varies frommercerisation to benzoylation. Similarly, in the third stage, the activationenergy increases from 210.82 to 332.51 kJ/mol as the as the fibre treatmentreaches to benzoylation. When the samples were compared, it was observedthat fibre treatment increases the interface bonding, which makes the systemmore thermally stable. tt is important to notice that the increase in activationenergy from stage I to It1 points to the fact that fibre filled Sl3R is morethermally stable than neat SBR. From Table 9.7, it is clear that the entropy ofactivation have negative values. 'Ihe magnitude of entropy is lower in thesecond stage in all cases.

9.2.5 DSC studiesIn DSC, the heat flow rate associated with a thermal event can be

measured as a function of time and temperature and it allows one to obtainquantitative information about melting and phase transition of the system.'lhey can yicld insight into various aspects of ~n~~tc~ri ' i ltructure, and canprovide a convenient measure of 'I,,: and may influence other in~portantproperties.

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'I'he thermal behaviour of sisal fibre, Sllli, sisal/SBR composites withand without bonding agent was analysed by DSC. Figure 9.16 shows the DSCcurve of raw sisal fibre. Figure 9.17 shows the DSC trace of the SBRcompound. The transition temperatures of composites observed are tabulatedin Table 9.9 along with those of component materials such as sisal and SBR.Table 9.9 showed the effect of volume of fibres and bonding agent on T,. T,sof sisal fibre and SUR gun1 compound are found to be 49 and -52.25'Crespectively. The T, value of SBR gum compound is found to be very close tothe already reported value irrespective of the method of preparation of mixes.Figure 9.18 shows the DSC trace of fibre filled composite (Mix E) containing5phr of fibres while Figure 9.19 shows the DSC traces of mix C containing35phr of short fibres. These DSC traces of composites show two endothermicpeaks indicating two different transitions. Conlpositc~s how two 'I',:. due tothe presence of fibres and rubber matrix.

-20 o m r#) 60 m i mTEMPERATURE ( O C )

Figu re 9.16. DSCcurve of raw sisal$bre.

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-100 -80 -60 -40 -20 0 20 40TEMPERATURE ( OC)

Figure 9.1 7. DSC curve o fg u m conlpourtd (SBR).

Table 9.9. Effect of variatiort of volume of fibres ($6) and irlcorporatiorl ofbonding agent o ft the glass transition tenrperature.Miwes

A

Chapter 9: Thetmal and Kinetic Degradation 28 8

L (withbonding agent)

Fibre length(mm)

-

6

Fibrecontent (%)

0

17.674 1 -24.6 76.5 I

'1'6 rc)of SBR-52.25

of sisal-

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-3I:-50J[r.2E

Chapter 9: Thermal and Kinet~c egradation 289

I I I I

-LI:-L0claI-dW1

u , - a j - 4 0 - a , o m 4 o m mTEMPERATURE ( OC)

Figure 9.18. DSC curve oflib refi lled coniposite(mk E).

, 9* ~ - l o ' - a , a, 40 a, mTEMPERATURE ( OC)

Figure 9.19. DSC curve of miu C coritainiizg 35pBr of fb r e s .

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Scvc-rnl rt-srnrrhcrs hilvc rc-portrd tlie chanp,c-s in 'I',: of the comyositcsupon the incorporation of fillers like fibres, particulate fillers, flakes, e t ~ " ~ ' ~ .On the addition of aramid, glass and carbon fibres to the polyphenylenesulphide, T, of the matrix was depressed by 5-3" 'Ihe incorporation ofbenzylated wood in polystyrene(PS) decreases the '1, f 1%" . lowering of'I', is due to the plasticisation effect of the fibre that diffuses or dissolves intothe polymer matrix.

Figure 9.20 shows the USC trace of the coniposite containing the 35phrof fibres and bonding agent. The two transitions are also noted from theseendothermic peaks indicating two different T,s of the composite, i.e., -24.6and +76.5. But here, the l',s show higher values compared with othercomposites. 'The high interfacial interaction between the fibre and matrix,which is created by the resin of the bonding agent, stiffens the composite andhence the TK value increase. ?'he hypothetical niechanisni showing tliereaction of the components with the bonding agent is depicted in Scheme 4.6.

a , - 4 0 - a , 0 m i t o m m 1 mTEMPERATURE ( OC)Figure 9.20. -DSC curve of mir L cotifairtirig bortdirig agerif.

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9.3 ReferencesJ. I..Laird and G. I.iolios, Arrrt2r. l..ob., H-16782 (Jan 1990).K. N. Niuaa, Alif). I'olyrrr. 'leclrrrol. Syrrrp. Proc. ' 9 6 , CUSAT, Kochi (1996).I. C. McNeill, '71vrrrml degrnttrfiorr, 111 Corrrprelretrsioe Polyrrrer Scierrce-6',G. Allen (Ed.), Pergamon press, New York (1989)Ch. 5.A. S. K~VIVC>II,11 " l ~ ~ ~ ~ l r r r i ~ j r r [ ~ s11r11 rrri~t/rc~~isf /i011/11r(*r~z~~r l r r t i f i~~rr ' ,ol I,eds. 1'. E.Slade jr. and I.. T. Jenkins, Marcel 'llekker, New York, U. S. A.(1966), p 217.S. C. Kitn , D. Kllempner, K. C. Frisch a i d H. L. I:risrl~, Aypl. Polyrrl. Sci., 21,1289 (1977).V. C. Belyakov, A. A. Berlin, I. I. Bukin, V. A. Orlov and 0 .G. Tarakanov,Polyrrr. sn. U S S R ,10,700 (1968).F. J. Kiljer, 'Higlr polllrrrers', Vo1. 5, Pt. V, Ed. N . hl . Bikales and L. Segar,Interscience, New ~ o ; k 1971) p 1015.Ivl. K. Sridllar, G. Hasavarajappa, S. G. Kasturi, N. Balasubramauan,ltrli. 7i2st.RF S . ,7,87 (1982).M. Jaffe, '7lrr.r-rrml cl~nmc-trrrarliorr of polyrrrenc rrrnf~rinls', Ed. E. A. Turi.,Academic press, Ne w York (1981)p 750.A. Basch and h.1. l.ewin, ). Polyrrr. Sci. - Polyrrr. Clrcrrr. Ed. , 11, 3071 (1973).A. Bascli and hl. Lewin, /. Polyrrr. Sci. - Poly~rr. ln-111. I.rl., 12, 2053 (1974).H. Rodrige, A. Basch and IvI. Lewin, I. Polyrlr. Sci. - Pc7l!yrrr. Clrerrr. Ed., 13, 1921(1975).A. Broicio, A. C. Javier-son, A. C. Quano, E. hl . Barrall, 1. A l ~p l .Polyrrr. Sn'., 17,3627 (1973).F. J. Abe, ]nplrr Wo oti Res. Sac., 14,98 (1968).D. N. Mahato, B. K. hlathur an d S. Battacharjee., 11rri. 1. Fib. tiTPX. t,s., 20, 202(1995).1 '. hl011.111lv.3. i ' r . ~ c l l ~ i ~ l l ,;. hlohilnty ilnd N. ~ I o ~ I ~ I I I ~ I . ,. ' l i . l r t l r . Kcs. ('111.111., 1(I), 11 (19~4) .L. Wall a i d j. Flurm, Rrrtrt1c.r Clrrrrr. Te(.lrrrol.,35, 1157 (1962).'1'. K. Chaki, A. K. Bhallacharya and 8.

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I?. T'ras;tntha Kuriiar, hl. I . . G i\nima, and S. ' I l ~ o ~ r ~ a s ,. Alil~ l . o l~/r r . i'i., 58,5'17 (IOC15).I